In the known methods of chronic blood purification therapy such as hemodialysis, hemofiltration or hemodiafiltration, the patient's blood is passed through an extracorporeal blood circuit which is generally equipped with a safety system that permanently monitors the arterial pressure and venous pressure within the circuit. The purpose of monitoring the pressure is to detect various complications that may arise during the extracorporeal blood treatment. Possible treatment complications include: an incorrect vascular access, which may be attributable for example to disconnection of the cannula or to suctioning of the cannula; loss of blood due to the cannula slipping out or due to leakage; kinking of the blood conduit, or coagulation in the blood conduit.
If an error state is detected in the extracorporeal circuit, the known safety systems are generally designed to stop the blood pump, to close the tube clamp in the venous blood conduit and to trigger an acoustic and/or optical warning signal. In this way, the blood treatment device is brought into a state which is safe for the patient, but which nevertheless leads to interruption of the therapy.
Various alarm systems are known with which error states in the extracorporeal blood circuit can be detected. The alarm systems are generally based on monitoring the pressure in the arterial and venous branches of the extracorporeal circuit. The known alarm systems infer the existence of an error state when predetermined limit values are exceeded or fallen below. In general, the decision concerning the error state depends only on whether the actually measured pressure does or does not lie within a predefined range.
The alarm systems based on pressure measurement generally respond when the arterial and/or venous pressure in the extracorporeal circuit exceeds or falls below a threshold value that can be varied by the hospital personnel between ±20 and ±60 torr. Alarm systems comparing the pressure values with threshold values have the advantage of being able to be rapidly adjusted. In the known systems, the pressure limit values above or below which an alarm is triggered can generally be input manually. These limit values are dependent both on the patient and on the type of treatment and are individually set at the start of each treatment.
With a large threshold range, there is of course the advantage of a low rate of false alarms, but large threshold ranges lead to a time delay between the detection of a patient-critical state and the response of the alarm system. There is additionally the danger of a patient-critical state no longer being able to be safely detected. By contrast, although a small threshold range increases patient safety, it necessarily leads to an increased rate of false alarms, and this can lead to desensitization of the monitoring personnel or even to manual deactivation of the alarm. The known alarm systems that monitor whether a threshold value is fallen below or exceeded have a rate of false alarms of 70-99.5% depending on the physiological parameter that is to be monitored (Wiklund L, Hök B, Stähl K, Jordeby-Jönsson A. Postanaesthesia monitoring revisited: Frequency of true and false alarms from different monitoring devices. J. Clin. Anesth. 1994; 6: 182-188).
The main causes of pressure-induced false alarms are, for example, movements of the patient, fluctuations in the patient's blood pressure, and fluctuations in the viscosity of the patient's blood.
If the patient changes his vertical position, for example if the patient's position is raised or lowered, the extracorporeal pressure rises or falls by about 0.75 torr per cm of height increase or decrease. This effect leads, for example, to the arterial and venous pressure dropping by about 10-20 torr if a patient moves from a seated position to a sleeping position. Since there is a height difference between puncture site and heart in a seated position, the fistula pressure when seated is higher than when lying down.
During night-time dialysis sessions, in particular in home dialysis, pressure-induced false alarms occur mostly when the patient enters the phase of deep sleep. The drop in blood pressure induced by deep sleep leads to a reduction in the pressure in the entire vascular access. By way of the arterial and venous dialysis cannulas, the drop in blood pressure leads to a symmetrical reduction in the arterial pressure and venous pressure in the extracorporeal circuit. Correspondingly, a rise in blood pressure leads to an increase in the arterial pressure and venous pressure.
Fluctuations in the viscosity of the patient's blood generally occur when, during the blood treatment, water is withdrawn from the patient by what is called ultrafiltration. By contrast, when the ultrafiltration rate is reduced, the viscosity of the patient's blood drops, since tissue water and cell water passes from the extracellular and intracellular volume into the blood volume.
In order to avoid false alarms caused by changes in the viscosity of the patient's blood, it is known to use adaptive drift detection algorithms which at fixed time intervals center the alarm limits arranged around the actual arterial and venous pressure values. For example, a constant increase in the viscosity of the patient's blood leads to a symmetrical increase in the flow resistance in the extracorporeal circuit, as a result of which the arterial pressure drops and the venous pressure rises. By contrast, a reduction in viscosity has the opposite effect. A disadvantage is that the known alarm systems with algorithms for adaptive drift detection can avoid only those alarms that result from a relatively slow change in the arterial and/or venous pressure in the extracorporeal circuit.
EP 1 584 339 A2 describes a device for monitoring a vascular access during dialysis treatment, in which the pressure is monitored by pressure sensors both in the arterial branch and in the venous branch of the extracorporeal blood circuit. From the arterial and venous pressure, two values characteristic of the state of the vascular access are calculated in a computing unit and are then evaluated in an evaluation unit for detection of an incorrect vascular access. The known monitoring system is based on calculation of two values that are characteristic of the state of the vascular access and that are compared to two threshold values, and it is inferred that the vascular access is incorrect if both the first and second characteristic values are negative and the first characteristic value is smaller than the first threshold value and the second characteristic value is smaller than the second threshold value.
To calculate the first characteristic value, the difference between the sum of the venous and arterial pressure of a subsequent measurement and the sum of the venous and arterial pressure of a preceding measurement is determined. Whereas, in order to calculate the second characteristic value, the difference between the difference of the venous and arterial pressure of a subsequent measurement and the difference of the venous and arterial pressure of a preceding measurement is determined. These pressure differences of consecutive measurements are continuously added up. Consequently, the monitoring of the vascular access is based on the comparison of measured values of the arterial and venous pressure that have been determined in preceding and subsequent measurements. Based solely on a single measurement of the arterial and venous pressure at a certain time, a possible error state cannot be detected using the known algorithm.
DE 101 59 620 C1 discloses a means for monitoring the delivery of substitution fluid during an extracorporeal blood treatment, where, instead of monitoring the pressure in the extracorporeal blood circuit as such, a disturbance in the delivery of substitution fluid is inferred when the amplitude of the pressure waves generated by the substituate pump exceeds a predetermined threshold value.
US 2002/0190863 A1 describes a medical monitoring system in which false alarms are intended to be ruled out by using the time sequences of two measurement parameters for the evaluation.